This invention relates to a system and method for generating energy from a variety of biomass feedstocks, and more particularly to a system for generating energy which a biomass gasifier system in conjunction with an integrated fuel cell.
Fuel cells have long been used in the space program to provide electricity and drinking water to astronauts. In the future, the electric power industry is expected to be an area where fuel cells will be widely commercialized. The electric power industry has generally been looking toward the use of fuel cells in relatively large electrical power generating applications. Power generation by fuel cells offers the advantages of high efficiency and low environmental emissions. Thus, fuel cells may offer a more economical means of power production than other existing power producing technologies.
Fuel cells produce electrical power by converting energy from the reaction of various products directly into electrical energy. An input fuel is chemically reacted in the fuel cell to create an electrical current. An electrolyte material is sandwiched between two electrodes, an anode and a cathode, making up the fuel cell. The input fuel passes over the anode, where it splits into ions and electrons. The electrons go through an external circuit to serve an electric load while the ions move through the electrolyte toward the oppositely charged electrode. At the electrode, ions combine to create by-products, primarily water and carbon dioxide. Depending on the type of electrolyte used in the fuel cell, different chemical reactions will occur.
For example, in some systems, hydrogen rich fuels and an oxidant gas, such as air are fed into a fuel cell stack, a series of electrode plates interconnected to produce a set voltage of electrical power. Typically, the hydrogen rich fuel gas is fed to the anode of the cell, while the cathode receives oxidant gas or air. Internal reforming of any hydrocarbons present in the fuel gas occurs at the anode. The reformed fuel gas in the anode compartment and the oxidant gas in the cathode compartment, in the presence of the electrolyte of the cell, undergo electrochemical conversion to generate electrical power.
There are several different types of fuel cells, the parameters of which can vary depending on what the cell will be used for, the structure of the cell and the materials used. These include proton exchange membrane fuel cells (PEFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC) and molten carbonate fuel cells, among others.
Molten carbonate fuel cells (MCFC) use a molten carbonate salt mixture as an electrolyte. The composition of the electrolyte varies, but may consist of lithium carbonate and potassium carbonate. At the operating temperature of about 1200xc2x0 F., the salt mixture is liquid and a good ionic conductor. The electrolyte is suspended in a porous, insulating and chemically inert ceramic (LiAlO2) matrix. The chemical reactions of the MCFC are as follows.
Solid oxide fuel cells (SOFC) use a ceramic, solid-phase electrolyte which reduces corrosion considerations and eliminates eletrolyte management problems sometimes associated with liquid electrolyte fuel cells. A preferred ceramic is yttria-stabilized zirconia, an excellent conductor of negatively charged oxide ions at high temperatures. The anode is preferably porous nickel/zirconia cement, while the cathode is preferably a magnesium-doped lanthanum manganate. The SOFC reactions are as follows.
Phosphoric acid fuel cells (PAFC) uses liquid phosphoric acid as the electrolyte. The acid is contained in a TEFLON bonded silicone carbide matrix, the small pore structure of which keeps the acid in place through capillary action. Platinum catalyzed, porous carbon electrodes are used on both the anode and the cathode sides of the electrolyte. The PAFC reactions that occur are as follows.
Proton exchange membrane fuel cells (PEFC) use a polymer membrane as the electrolyte. The membrane is an electronic insulator, but an excellant conductor of hydrogen ions. The PEFC membrane consists of fluorocarbon polymer materials, for example TEFLON, to which sulfonic acid groups are attached. The protons on these acid groups are free to migrate through the membrane. Platinum is used at both the anode and the cathode.
The electrode reactions in the PEFC are analogous to those in the PAFC, and are as follows.
Molten carbonate fuel cells and solid oxide fuel cells are well suited for using heated gas streams and, thus, have shown the most promise in industrial power generation applications. There are several known sources for fuel gas suitable for use in these fuel cells. Natural gas may be used as a fuel, although it may be necessary to use a fuel processor to boost the concentration of hydrogen present in the natural gas. Fuel gas may also generated in coal gasifiers, which generate hydrogen, carbon monoxide and carbon dioxide has also been found suitable for use as a fuel gas to feed fuel cells. Additionally, biomass gasifiers are also known in the art and have been found useful for the production of fuel gases in remote areas or in areas wherein a large amount of agricultural biomass waste is produced.
Greater efficiency in conventional fuel cells may be obtained through integration with coal or biomass gasifiers. For example, U.S. Pat. No. 4,921,765 to Gmeindl et al. discloses a combined gasifier and fuel cell system wherein the gas stream travels from the gasifier through an external carbon dioxide separator. In the Gmeindl et al. fuel cell system, the anode reaction gases are recycled to provide the steam and heat needed to support the gasifier. The process disclosed in the Gmeindl process uses coal or coal char to feed the system.
U.S. Pat. No. 5,554,453 to Steinfeld et al. discloses a carbonate fuel cell system with thermally integrated gasification. The system disclosed by Steinfeld uses a portion of the output gas from a gasifier as the fuel gas for a molten carbonate fuel cell (MCFC). The remainder of the output gas is combusted to provide heat for driving the gasification reaction and to produce a CO2 rich exhaust gas. The CO2 rich exhaust gas is mixed with air and used as the oxidant gas at the cathode of the fuel cell. Steinfeld discloses system configurations, one wherein a catalytic combustor is situated within the gasifier and the other with a catalytic combustor situated externally to the gasifier. Each of Steinfeld""s fuel cell systems require either hot or cold gas clean-up, followed by expansion to provide moisturization of the gas. The Steinfeld et al. fuel cell system may be suitable for use with either a coal gasifier or with some biomass gasifiers.
Biomass gasification systems known in the art generally rely on combustion of a portion of the biomass feedstock to provide the heat required for gasification of the remainder of the biomass feedstock. However, the combustion of a portion of the raw biomass stream for heat production can significantly reduce the overall efficiency of the gasifier system. As a result, these systems generally operate at an efficiency of less than 25% overall conversion efficiency to electrical power.
Higher efficiencies, approaching 60% have been achieved using the combustion of natural gas to provide heat for the gasification process, however, natural gas is not always readily available. It has also proven advantageous to utilize the waste carbonaceous char produced in the gasification as a fuel source for generating heat in a combustor. Since the char is basically a waste product from the gasifier, its consumption in the combustor has less of an adverse effect on the system efficiency than is seen in systems wherein a portion of the raw biomass is used as a combustor fuel source.
U.S. Pat. No. 4,828,581 to Feldmann et al., describes an exemplary gasifier system for the production of fuel grade gas from carbonaceous fuels using very high biomass throughputs in a fluidized bed gasifier operating at low inlet gas velocities. The process described in Feldmann et al. uses a combustor to heat a bed of fluidized sand, which is directed to a gasifier wherein the heated sand serves as a heat source for the pyrolysis of the biomass material. Unlike prior systems, the system of Feldmann et al. relies on the entrainment of char in a flow of sand from the gasifier outlet to the combustor to allow operation at an advantageously low inlet velocity. The Feldman et al. system is suited to the production of a medium BTU gas which may be used as a fuel source in a fuel cell system.
The biomass gasification system described in Feldman also has the advantage of being adaptable to relatively small scale applications. Generally, due to heat loss considerations, the efficiency of biomass gasifiers increases with increasing input of feedstock material. At decreasing inputs, prior art systems reach a point at which the percentage of heat loss increases exponentially, effectively limiting these prior systems to inputs of greater than approximately 100 tons per day. If throughput is defined as the ratio of input to cross section, then at high throughputs the ratio becomes less favorable and requires a higher overall system input to maintain an acceptable level of efficiency. Accordingly, prior to the development of the Feldman system, many systems were limited to operation at feedstock input rates of greater than approximately 100 tons per day.
However, there are many applications wherein in it is impractical to maintain high feedstock input rate on the order of 100 tons per day, such as to provide power to small communities or industrial facilities having low power requirements. It would clearly be desirable to operate these systems at a higher throughput because the resulting gasifier unit could be both smaller and cheaper to construct than a conventional low-throughput gasifer of the same capacity. Prior conventional gasifier systems have required a tradeoff between unit cost and efficiency.
Accordingly, it is an object of the present invention to provide an integrated biomass gasification and fuel cell system having a gasifier capable of operating at a wide range of feed rates such as from 20 to 1000 tons per day, or greater.
Another object of the present invention is to provide an integrated biomass gasification and fuel cell system which operates at a high temperatures, thus increasing the quantity of product gas produced per unit of biomass fed and increasing overall efficiency of energy production.
It is yet another object of the present invention to provide an improved integrated biomass gasification and fuel cell system wherein anode offgas is recycled and used to produce heat to provide increased efficiency of energy production.
The present invention relates to a system and method for efficient energy generation from a variety of biomass feedstocks forms by integrating a fuel cell into a highly efficient parallel entrained bed pyrolysis gasification system. Gas is produced using a high throughput combination gasifier and combustor, wherein the exothermic combustion reactions can take place in or near the combustor while the endothermic gasification reactions take place in the gasifier. Heat from the exothermic reaction zone of the combustor is transferred to the endothermic reaction zone of the gasifier by circulation of an inert particulate solid such as sand. This separation of endothermic and exothermic processes results in a high energy density product gas without the nitrogen dilution present in conventional air-blown gasification systems.
The fuel cell utilizes the product gas generated by the gasifier as its anode gas. At least a portion of the exhaust gas from the anode is then routed to the combustor wherein it is combusted to recover a portion of its residual energy in the form of heat. By using the combustion of the anode exhaust gas to heat the gasifier, overall system efficiency can be increased. Oxidant gas from the combustor may also be directed to the cathode of the fuel cell.